1. Field of the Invention
The present invetion relates to novel human C-peptide derivatives, and, more particularly, relates to tyrosylated human C-peptide derivatives and radioactive iodized derivatives thereof. These C-peptide derivatives are useful for the radioimmunoassay of human C-peptide or proinsulin in serum.
2. Description of the Prior Art
Human proinsulin, which is a precursor of insulin, is known to have the structure as NH.sub.2 -(B-chain of insulin)-Arg-Arg-(C-peptide)-Lys-Arg-(A-chain) (Proc. Nat. Acad. Sci., Vol. 67, pages 148 to 155 (1970)). As the blood level of human C-peptide is relative to those of proinsulin and insulin, quantitative analysis of C-peptide, especially by a radioimmunoassay method, is of great importance in clinical analysis.
Steiner at al reported that human C-peptide is extracted from human pancreas and also proposed its amino acid sequence as Glu-Ala-Glu-Asp-Leu-Gln-Val-Gly-Gln-Val-Glu-Leu-Gly-Gly-Gly-Pro-Gly-Ala-Gl y-Ser-Leu-Gln-Pro-Leu-Ala-Leu-Glu-Gly-Ser-Leu-Gln, (J. Biol. Chem., Vol. 246, pages 1375 to 1386 (1971)).
Further, Steiner et al reported the immunoassay method using .sup.131 I-tyrosylated C-peptide which is prepared from the extracted C-peptide (Diabetes, Vol. 19, pages 546 to 551, and Proc. Nat. Acad. Sci., Vol. 67, pages 148).
In the process of immunoassay, antiserum containing antibody to human C-peptide is necessary and the antiserum is generally produced in animals such as guinea pigs or rabbits with antigen, i.e., C-peptide. Usually, the antiserum is required to have an adequate antibody unit so high as being measurable even after high dilution, desirably 3,000 to 5,000 times dilution, in order to avoid the undesirable effect of various kinds of inevitable contaminants in the serum.
The antiserum given from C-peptide in Steiner's report enables measurement with 1,000 times dilution, however, this is not yet satisfactory. Moreover, the extraction of C-peptide from human pancreas can not be carried out on a large scale, and C-peptide is not obtained in high yield by a synthetic procedure because it is slightly soluble in an aqueous buffer solution.